The specific excitation of mercury isotopes by photochemical means is well established. See Webster, C. R. and Zare R. W., Photochemical Isotope Separation of Hg-196 by Reaction with Hydrogen Halides, J. Phys. Chem. 85, 1302 (1981).
Mercury vapor lamps are commonly used as the excitation source of Hg isotope specific photochemical reactions. To be successful, photochemical separation of a single isotope requires that the spectral bandwidth of the exciting mercury lamp or laser source must be sufficiently narrow to excite only the isotope of interest, the specificity depending on the spectral bandwidth of the source. The rate and extent of separation of the particular isotope from the feedstock can be strongly dependent on the intensity of the radiation emitted from the mercury lamp.
The vapor equilibrium pressure of the Hg used in the mercury lamp strongly affects the intensity and spectral linewidth of the light which is emitted from the lamp. Lamps of the prior art used for this purpose are not able to adequately control the Hg vapor pressure inside of the lamps. This is due to the fact that the lamp cold spot is not well established. The lamp cold spot is the lowest temperature region within a lamp. This cold spot temperature determines the Hg equilibrium vapor pressure within the lamp. After lamp start-up, many hours of lamp operation may be required to fix the region. During this transition time, a definite Hg pressure is not attained. This variance in the vapor pressure of the mercury within the lamp can cause disturbances in the linewidth and intensity of resonance radiation emitted, thus, undersirable isotopes of Hg can be stimulated and the rate of separation of the desired isotope of mercury can be affected. Further, without knowing the location of the cold spot, it may not be possible to monitor the Hg vapor pressure.